The Following Data Show the Total Output for a Firm

The following data show the total output for a firm when different amounts of labour are combined with a fixed amount of capital. Assume that the wage per unit of labour is $10 and the cost of the capital is $50. $$\begin{array}{l}\begin{array} { | l | l | } \hline \text { Labour per period } & \text { Total Output per period } \\\hline 0 & 0 \\\hline 1 & 10 \\\hline 2 & 30 \\\hline 3 & 90 \\\hline 4 & 132 \\\hline 5 & 150 \\\hline\end{array}\\\text { TABLE 7- } 3\end{array}$$
-Refer to Table 7- 3. If this firm is producing 20 units of output per period its marginal cost is

A) $1.67.
B) $10.00.
C) $1.00.
D) 50 cents.
E) 16.7 cents.